Leiden ESA Astrophysics Program for Summer Students 

Emma Dodd

My Experience

This summer I was lucky enough to be able to undertake a research project at ESTEC (European Space Research and Technology Centre) in Noordwijk, Netherlands.


I stayed in Leiden which is a beautiful city and got to visit Leiden University also!


It is a wonderful university for physics, some Nobel laureates associated with Leiden include Albert Einstein, Enrico Fermi and Paul Ehrenfest. It also has the oldest university observatory in operation today, which was established in 1633, and we were lucky enough to have a tour and look at Mars, Jupiter and the Moon through the telescopes whilst watching the lunar eclipse on 27th July.


The city contains six wall murals of physics equations dotted around to celebrate science in the city and we had a fun time exploring the city to find them all. Here is an example of Einstein field equations depicting gravitational lensing.


We had a talk each week at Leiden University on a huge range of topics. My favourite, by Jorryt Matthee, on ‘The formation of galaxies in the first 3 Gyrs’, even mentioned about CR7.

We would also attend a talk at ESTEC on a Friday, and these were incredibly interesting! There was one talking about the trajectory of JUICE, an ESA probe that will be launched to study Jupiter’s moons in 2022, and another talking about Mars’s magnetic field and how spherical harmonics are used to find the magnetisation.

This year there were 18 students in LEAPS, all working on a range of different projects and all from different scientific backgrounds at different levels of study. I met people from all over the world and made some really good friends!


While I was in the Netherlands I rented a bike as it is so convenient for getting around, as everywhere is so flat! My bike was a back-pedal brake bike which was strange at first but now I couldn’t imagine having a ‘normal’ hand brake bike and will miss cycling in such a beautiful area.

One of the few words I learnt in Dutch was ‘borrel’ which translates to English as ‘a drink’ but is used to mean an informal social gathering with beers! We had many of these! We managed to find an English pub quiz over there which we went to every week, even if we didn’t do very well!

We also got the chance to visit many other places whilst in the Netherlands including a tour of ESTEC with all the LEAPS students, a visit to Westerbork radio telescope and to LOFAR.

Another highlight was that I even got to try liquid nitrogen ice cream! I have had such a wonderful time here over the last two months and am so grateful for this opportunity I was given.


My Research

Now for the science I have been doing!


When stars form they form in clusters and there are two different types of these clusters. Open clusters are young, loosely bound clusters, usually associated with star forming regions. Globular clusters are much older and contain many more stars. They are massive and dense enough to be held together by their own gravity and are usually approximated to be spherical. The nearest star to the sun is 4 light years away, whereas, in the dense core of globular clusters, stars can be fractions of light years apart and so if we were to be on a planet orbiting one of these stars, the night sky would be so bright it might even be possible to read by starlight.

Screenshot 2018-08-14 10.35.00

Within our own galaxy, open clusters can be found in the spiral arms, in the regions where stars are still forming and globular clusters in the halo orbiting the galactic centre, are no longer forming. There are around 150 known globular clusters in our galaxy and it was by studying the distribution of these clusters that astronomers first started to suspect that the sun was not at the centre of the galaxy.

So, globular clusters contain old stars, but how do we know this? Since all stars in a cluster formed at the same time, it is possible to determine the age of a cluster by looking at the age of the stars themselves. This can be done by looking at images of the cluster in two different filters, then by looking at the difference in magnitudes of the stars you can use this to plot a colour magnitude diagram and since the hot blue stars will die off first, then by looking at the most massive stars still left in the cluster (the turn off point) you can find its age.


This is found to be of the same order of magnitude of the age of the universe, suggesting these clusters were the building blocks of galaxies and so by studying them and developing our understanding in their dynamics we can learn more about their host systems.

My goal was to, with the help and supervision from Alice Zocchi, study the dynamics of globular clusters to understand what are the main ingredients that shape their properties. The spherical approximation is too simple as some clusters are flattened and there are different possible causes for this; rotation, effect of tides and velocity anisotropy.

I started looking at two star clusters in which the data was generated from models and so it was possible to have all x, y, z positions and the corresponding velocity components. In reality, this will never be the case as from observation we see the clusters as a 2D projection on the plane of the sky. I started by investigating the different line of sights, for a spherical cluster (red) and a flattened cluster (blue), to see how the projection changes. I looked at how it isn’t possible, with only the one view, from observations, to conclude if a cluster is either flattened or perfectly spherical. And so, I needed to find other ways to see if a cluster is flattened.

Screenshot 2018-08-13 14.35.34Screenshot 2018-08-13 14.35.50

First, I looked at the density and how that differs due to the flattening. I then looked at the rotation in the clusters and this is what I spent most of my time focusing on. Stars in a cluster either orbit in random chaotic motion, which counteracts the gravity and stops the cluster flattening, or in organised motion which causes an overall rotation of the cluster. I looked at how the rotation will look in different projections and also how polar coordinates can be better for tracking rotation and compression/expansion of a cluster. I also saw how the velocity dispersion changes when a cluster rotates.

Next, I began to work on real observational data, taken with FLAMES, the spectrograph on the VLT in Chile, which measures the Doppler shift of stars to give line of sight velocities.

Screenshot 2018-08-14 10.46.07

The globular cluster I was looking at in detail was NGC 6541 that located in the southern constellation of Corona Australis and has an eccentricity of 0.12 but the dynamics have not yet been studied in detail, before now.

The Hubble Legacy Archive

Since clusters are randomly orientated in the sky, the rotation axis of a cluster will not necessarily line up with a coordinate axis and so we have to find the position angle of the cluster, which is the angle of the projected rotation axis on the plane of the sky. This is done by looking at difference of the average velocity of the stars in each half of the cluster and the maximum difference corresponds to the position angle where zero is when the line splitting the cluster in half is in the y axis.


From this I was able to plot a rotation curve for this cluster amongst other things and then go about fitting models to the data to find out more about this cluster.


In the last week, we all presented our work on the variety of topics we had been studying and went for a final meal before saying our goodbyes.

I have enjoyed every aspect of the last two months and have learnt an incredible amount, not only just physics but also about myself as a person. Going abroad seemed so daunting running up to the first day, but I would recommend everyone to try something like this as I have met so many amazing people from all over the world, learnt more than I could have imagined and got to travel and see new places at the same time.